3.6 Options Controlling Objective-C and Objective-C++ Dialects

This section describes the command-line options that are only meaningful
for Objective-C and Objective-C++ programs, but you can also use most of
the language-independent GNU compiler options.
For example, you might compile a file some_class.m like this:

gcc -g -fgnu-runtime -O -c some_class.m

In this example, -fgnu-runtime is an option meant only for
Objective-C and Objective-C++ programs; you can use the other options with
any language supported by GCC.

Note that since Objective-C is an extension of the C language, Objective-C
compilations may also use options specific to the C front-end (e.g.,
-Wtraditional). Similarly, Objective-C++ compilations may use
C++-specific options (e.g., -Wabi).

Here is a list of options that are only for compiling Objective-C
and Objective-C++ programs:

-fconstant-string-class=class-name

Use class-name as the name of the class to instantiate for each
literal string specified with the syntax @"...". The default
class name is NXConstantString if the GNU runtime is being used, and
NSConstantString if the NeXT runtime is being used (see below). The
-fconstant-cfstrings option, if also present, will override the
-fconstant-string-class setting and cause @"..." literals
to be laid out as constant CoreFoundation strings.

-fgnu-runtime

Generate object code compatible with the standard GNU Objective-C
runtime. This is the default for most types of systems.

-fnext-runtime

Generate output compatible with the NeXT runtime. This is the default
for NeXT-based systems, including Darwin and Mac OS X. The macro
__NEXT_RUNTIME__ is predefined if (and only if) this option is
used.

-fno-nil-receivers

Assume that all Objective-C message dispatches (e.g.,
[receiver message:arg]) in this translation unit ensure that the receiver
is not nil. This allows for more efficient entry points in the runtime
to be used. Currently, this option is only available in conjunction with
the NeXT runtime on Mac OS X 10.3 and later.

-fobjc-call-cxx-cdtors

For each Objective-C class, check if any of its instance variables is a
C++ object with a non-trivial default constructor. If so, synthesize a
special - (id) .cxx_construct instance method that will run
non-trivial default constructors on any such instance variables, in order,
and then return self. Similarly, check if any instance variable
is a C++ object with a non-trivial destructor, and if so, synthesize a
special - (void) .cxx_destruct method that will run
all such default destructors, in reverse order.

The - (id) .cxx_construct and/or - (void) .cxx_destruct methods
thusly generated will only operate on instance variables declared in the
current Objective-C class, and not those inherited from superclasses. It
is the responsibility of the Objective-C runtime to invoke all such methods
in an object's inheritance hierarchy. The - (id) .cxx_construct methods
will be invoked by the runtime immediately after a new object
instance is allocated; the - (void) .cxx_destruct methods will
be invoked immediately before the runtime deallocates an object instance.

As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
support for invoking the - (id) .cxx_construct and
- (void) .cxx_destruct methods.

-fobjc-direct-dispatch

Allow fast jumps to the message dispatcher. On Darwin this is
accomplished via the comm page.

-fobjc-exceptions

Enable syntactic support for structured exception handling in Objective-C,
similar to what is offered by C++ and Java. This option is
unavailable in conjunction with the NeXT runtime on Mac OS X 10.2 and
earlier.

The @throw statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a @catch block, the
@throw may appear without an argument (as shown above), in which case
the object caught by the @catch will be rethrown.

Note that only (pointers to) Objective-C objects may be thrown and
caught using this scheme. When an object is thrown, it will be caught
by the nearest @catch clause capable of handling objects of that type,
analogously to how catch blocks work in C++ and Java. A
@catch(id ...) clause (as shown above) may also be provided to catch
any and all Objective-C exceptions not caught by previous @catch
clauses (if any).

The @finally clause, if present, will be executed upon exit from the
immediately preceding @try ... @catch section. This will happen
regardless of whether any exceptions are thrown, caught or rethrown
inside the @try ... @catch section, analogously to the behavior
of the finally clause in Java.

There are several caveats to using the new exception mechanism:

Although currently designed to be binary compatible with NS_HANDLER-style
idioms provided by the NSException class, the new
exceptions can only be used on Mac OS X 10.3 (Panther) and later
systems, due to additional functionality needed in the (NeXT) Objective-C
runtime.

As mentioned above, the new exceptions do not support handling
types other than Objective-C objects. Furthermore, when used from
Objective-C++, the Objective-C exception model does not interoperate with C++
exceptions at this time. This means you cannot @throw an exception
from Objective-C and catch it in C++, or vice versa
(i.e., throw ... @catch).

The -fobjc-exceptions switch also enables the use of synchronization
blocks for thread-safe execution:

@synchronized (ObjCClass *guard) {
...
}

Upon entering the @synchronized block, a thread of execution shall
first check whether a lock has been placed on the corresponding guard
object by another thread. If it has, the current thread shall wait until
the other thread relinquishes its lock. Once guard becomes available,
the current thread will place its own lock on it, execute the code contained in
the @synchronized block, and finally relinquish the lock (thereby
making guard available to other threads).

Unlike Java, Objective-C does not allow for entire methods to be marked
@synchronized. Note that throwing exceptions out of
@synchronized blocks is allowed, and will cause the guarding object
to be unlocked properly.

Emit a special marker instructing ld(1) not to statically link in
the resulting object file, and allow dyld(1) to load it in at
run time instead. This is used in conjunction with the Fix-and-Continue
debugging mode, where the object file in question may be recompiled and
dynamically reloaded in the course of program execution, without the need
to restart the program itself. Currently, Fix-and-Continue functionality
is only available in conjunction with the NeXT runtime on Mac OS X 10.3
and later.

-fzero-link

When compiling for the NeXT runtime, the compiler ordinarily replaces calls
to objc_getClass("...") (when the name of the class is known at
compile time) with static class references that get initialized at load time,
which improves run-time performance. Specifying the -fzero-link flag
suppresses this behavior and causes calls to objc_getClass("...")
to be retained. This is useful in Zero-Link debugging mode, since it allows
for individual class implementations to be modified during program execution.

-gen-decls

Dump interface declarations for all classes seen in the source file to a
file named sourcename.decl.

-Wassign-intercept (Objective-C and Objective-C++ only)

Warn whenever an Objective-C assignment is being intercepted by the
garbage collector.

-Wno-protocol (Objective-C and Objective-C++ only)

If a class is declared to implement a protocol, a warning is issued for
every method in the protocol that is not implemented by the class. The
default behavior is to issue a warning for every method not explicitly
implemented in the class, even if a method implementation is inherited
from the superclass. If you use the -Wno-protocol option, then
methods inherited from the superclass are considered to be implemented,
and no warning is issued for them.

-Wselector (Objective-C and Objective-C++ only)

Warn if multiple methods of different types for the same selector are
found during compilation. The check is performed on the list of methods
in the final stage of compilation. Additionally, a check is performed
for each selector appearing in a @selector(...)
expression, and a corresponding method for that selector has been found
during compilation. Because these checks scan the method table only at
the end of compilation, these warnings are not produced if the final
stage of compilation is not reached, for example because an error is
found during compilation, or because the -fsyntax-only option is
being used.

-Wstrict-selector-match (Objective-C and Objective-C++ only)

Warn if multiple methods with differing argument and/or return types are
found for a given selector when attempting to send a message using this
selector to a receiver of type id or Class. When this flag
is off (which is the default behavior), the compiler will omit such warnings
if any differences found are confined to types which share the same size
and alignment.

-Wundeclared-selector (Objective-C and Objective-C++ only)

Warn if a @selector(...) expression referring to an
undeclared selector is found. A selector is considered undeclared if no
method with that name has been declared before the
@selector(...) expression, either explicitly in an
@interface or @protocol declaration, or implicitly in
an @implementation section. This option always performs its
checks as soon as a @selector(...) expression is found,
while -Wselector only performs its checks in the final stage of
compilation. This also enforces the coding style convention
that methods and selectors must be declared before being used.

-print-objc-runtime-info

Generate C header describing the largest structure that is passed by
value, if any.